Together with drastic increase of the Internet demand, grappling for achievement of a very high speed and a very great capacity is being actively carried out in optical communication/optical transmission.
Particularly, a semiconductor laser which can carry out direct modulation at 25 Gb/s or more is required for an ultra-high speed optical fiber transmission system for 40 Gb/s or more or for a data com such as, for example, the 100 Gb/s Ethernet (registered trademark) wherein four waves of 25 Gb/s are bundled by WDM (Wavelength Division Multiplexing).
As a semiconductor laser which can implement such high-speed direct modulation, a DFB (Distributed Feed-Back) laser is expected.
Basically, in a semiconductor laser, if the volume of an active layer is decreased as far as possible, then the value of the relaxation oscillation frequency increases, and the bit rate with which direct modulation can be carried out increases.
Actually, some DFB laser has implemented 40 Gb/s modulation at a room temperature by setting the length of a cavity of a DFB laser as short as 100 μm.
However, in such a DFB laser as described above, as illustrated in FIG. 28, an anti-reflection coating (reflection preventing film) is provided on a front end face and a high-reflection coating (high reflective film; reflectance of approximately 90%) is provided on a rear end face, and a diffraction grating which does not have a phase shift is provided along an active layer.
Therefore, the yield of devices with which single-longitudinal-mode oscillation is obtained depends much upon the phase of the diffraction grating on the rear end face. Then, the period of the diffraction grating is as fine as approximately 200 nm, and it is substantially impossible to precisely control the position of the end face upon cleavage into devices. Therefore, the phase on the rear end face is obliged to become random. Accordingly, the yield of devices with which good single-longitudinal-mode oscillation is obtained cannot be raised.
Further, a distributed reflector type (DR) laser, which achieves enhancement of the yield of devices with which single-longitudinal-mode oscillation is obtained and can carry out high-efficiency laser operation, is also available. Such a distributed reflector type laser as just described has, as a reflector on the rear end face thereof, not a high-reflection film but a diffraction grating having the same period as that of a diffraction grating in an active region and includes a passive region which functions as a passive reflector. Further, a phase shift is provided between the active region and the passive region. It is to be noted that the period of the diffraction grating in the passive region is fixed.
Also a distributed reflector type laser is available wherein, in order to raise the reflectance in the passive region, the depth of grooves of the diffraction grating in the passive region is set greater than that of grooves of the diffraction grating in the active region or the equivalent refractive index in the passive region is made different. It is to be noted that, also where the depth of grooves of the diffraction grating in the passive region is set greater than that of grooves of the diffraction grating in the active region, in the passive region, the period of the diffraction grating is fixed and also the depth of grooves of the diffraction grating is fixed. Moreover, where the equivalent refractive index in the passive region is made different with respect to the depth of grooves of the diffraction grating in the active region, in the passive region, the period of the diffraction grating is fixed and also the equivalent refractive index is fixed.
Furthermore, a distributed reflector type laser is available wherein a λ/4 phase shift is provided at the center of the active region and a passive region having a diffraction grating is provided as a reflector on the rear end face and besides the pitch of the diffraction grating is made different between the active region and the passive region in accordance with the difference in composition between the active region and the passive region. Consequently, the optical pitches of the diffraction gratings in the active region and the passive region are made equal to each other and the Bragg reflection wavelength in the passive region is made equal to the wavelength of laser light generated in the active region so that the laser light can be reflected effectively. It is to be noted that, although the pitch of the diffraction grating in the passive region is made different from the pitch of the diffraction grating in the active region, the pitch of the diffraction grating is fixed in the passive region. Also, the optical pitch of the diffraction grating in the passive region is fixed.
Furthermore, a distributed reflector type laser is available wherein, by providing a transition region in which the composition varies between the active region and the passive region such that the pitch of the diffraction grating is varied in conformity with the variation of the composition, the optical pitches (optical lengths of diffraction grating pitches) of the diffraction grating are made equal to each other between the active region and the transition region. It is to be noted that, while the pitch of the diffraction grating varies in the transition region, the optical pitch is fixed.